Halios Christos H, Barlow Janet F
Department of Meteorology, University of Reading, Earley Gate, PO Box 243, Reading, RG6 6BB UK.
Boundary Layer Meteorol. 2018;166(3):395-422. doi: 10.1007/s10546-017-0300-z. Epub 2017 Oct 29.
The study of the boundary layer can be most difficult when it is in transition and forced by a complex surface, such as an urban area. Here, a novel combination of ground-based remote sensing and in situ instrumentation in central London, UK, is deployed, aiming to capture the full evolution of the urban boundary layer (UBL) from night-time until the fully-developed convective phase. In contrast with the night-time stable boundary layer observed over rural areas, the night-time UBL is weakly convective. Therefore, a new approach for the detection of the morning-transition and rapid-growth phases is introduced, based on the sharp, quasi-linear increase of the mixing height. The urban morning-transition phase varied in duration between 0.5 and 4 h and the growth rate of the mixing layer during the rapid-growth phase had a strong positive relationship with the convective velocity scale, and a weaker, negative relationship with wind speed. Wind shear was found to be higher during the night-time and morning-transition phases than the rapid-growth phase and the shear production of turbulent kinetic energy near the mixing-layer top was around six times larger than surface shear production in summer, and around 1.5 times larger in winter. In summer under low winds, low-level jets dominated the UBL, and shear production was greater than buoyant production during the night-time and the morning-transition phase near the mixing-layer top. Within the rapid-growth phase, buoyant production dominated at the surface, but shear production dominated in the upper half of the UBL. These results imply that regional flows such as low-level jets play an important role alongside surface forcing in determining UBL structure and growth.
当边界层处于过渡状态并受到复杂表面（如市区）的强迫作用时，对其进行研究可能极具难度。在此，英国伦敦市中心部署了一种基于地面的遥感与现场仪器的新型组合，旨在捕捉城市边界层（UBL）从夜间直至充分发展的对流阶段的完整演变过程。与在农村地区观测到的夜间稳定边界层不同，夜间城市边界层的对流较弱。因此，基于混合层急剧、近似线性的增加，引入了一种检测早晨过渡和快速增长阶段的新方法。城市早晨过渡阶段的持续时间在0.5至4小时之间变化，快速增长阶段混合层的增长率与对流速度尺度呈强正相关，与风速呈较弱的负相关。研究发现，夜间和早晨过渡阶段的风切变高于快速增长阶段，夏季混合层顶部附近湍流动能的切变产生约为表面切变产生的六倍，冬季则约为1.5倍。在夏季微风条件下，低空急流主导城市边界层，夜间和早晨过渡阶段混合层顶部附近的切变产生大于浮力产生。在快速增长阶段，表面浮力产生占主导，但在城市边界层上半部分切变产生占主导。这些结果表明，诸如低空急流等区域气流在决定城市边界层结构和增长方面，与表面强迫作用同样重要。
